The interior face of an automobile roof is commonly covered or lined with material which presents an attractive appearance and also acts as a sound absorber. Molded fiber glass panels and foam liners are examples of such liners. Basically, these products adequately perform the functions for which they were designed, but they are too expensive to be installed in economy automobiles. The economy class requires a liner that is not only attractive and provides an acoustical function, but is also considerably less costly.
One of the materials designers usually consider when confronted with the need to produce an inexpensive shaped product is wood fiber. This is because wood fibers are readily available at relatively low prices and can be formed into various shapes by a variety of manufacturing techniques. Although the use of headliners formed from wood fibers is not new, previous attempts to make a product acceptable to today's market have failed. Hardboard liners, for example, have been manufactured from a wood fiber mat formed from an aqueous fiber slurry. The mat included suitable resins to enable it to be densified and shaped under heat and pressure. The resulting product was too heavy, was difficult to mold, and had poor acoustical properties.
In efforts to make lighter liners and to reduce costs, attempts have been made to produce liners from other forms of wood fiber products. Kraft paper has been used to produce liners, as has corrugated paperboard formed from a corrugated medium sandwiched between two paperboard facers. In general, however, the resulting products, unless modified severely, lack adequate sound absorbing qualities and are difficult to form into the dished shaped of a liner without cracking.
An example of a headliner formed from corrugated paperboard is disclosed in U.S. Pat. No. 4,119,451. This patent describes a liner comprised of single corrugated paperboard which must be formed with notches in areas of sharp curvature. The width of the notches increases during molding to disperse molding stresses in order to avoid the formation of cracks. The patent is not concerned with the acoustical performance of the liner and does not suggest ways to increase the overall strength of the liner through use of a laminate other than a single corrugated base paperboard sheet.
Another example of a paperboard liner is disclosed in U.S. Pat. No. 4,131,702. The liner in this patent is encased in a foam layer on all sides in order to prevent loss of rigidity due to contact with moisture. No attempt was made to improve the acoustical performance of the liner apart from any acoustical benefit which may be provided by the foam material. Although a double corrugated liner is shown in the drawing, the description does not go into any details as to how the double layer is arranged with respect to the liner, nor does it comment on any of the parameters required of the double corrugated base member. It is clear from the disclosure of the patent that the paperboard unit, regardless of its construction, must be entirely encased in foam material.
Obviously, it would be highly desirable to have an automotive headliner of attractive appearance which could be formed from wood fibers, which possesses the beneficial qualities of a headliner discussed above, and which need not be limited to the designs of prior art paperboard liners.